1. Field of the Invention
The present invention relates generally to scanning radio-frequency direction-finding arrangements, and more particularly to such systems employing sum and difference beam patterns.
2. Description of the Prior Art
Such direction-finding arrangements are known in prior art technical literature, e.g., the text "Radar Handbook" (McGraw-Hill, 1970), P. 21-13, (by Merrill Skolnik), where in connection with a monopulse tracking radar a so-called difference pattern, having a minimum in the symmetry (boresite) axis, and a so-called sum pattern, having a maximum in the symmetry (boresite) axis, are used. In such direction-finding arrangements, which are employed to determine the direction of radiating targets, large multipath errors may be caused by the superposition of the radiation coming directly from the target and the indirect radiation arriving at the direction finder by way of interfering reflectors (the ground for example). If direction finder and target have only a small height above ground, angle measurements (in the vertical plane mainly) will be disturbed by the so-called specular reflection.
As shown in FIG. 1, during radar elevation measurements, the earth's surface acts as a reflector. The reflecting cross-section of the earth's surface can become very large, causing the indirect radiation I travelling from the target Z to a direction-finding antenna by way of the earth's surface to be of the same order of strength as the useful or direct radiation D. Such reflection interference, which is sometimes called "specular reflection" because the interference seems to emanate from the mirror image Z' of the target Z (FIG. 1), causes unacceptably large direction-finding errors.
It is known to reduce the influence of multipath interference on the direction-finding accuracy by putting obstacles in the way of the indirect radiation and thereby intercepting it. In the radar arrangement described in Vol. 3 of "Die Funkmessgerate der deutschen Flakartillerie" of the Bucherei fur Funkortung, Verkehrsund Wirschaftsverlag GmbH Dortmund, pp. 117 to 119, for example, attempts were made to suppress the rays reflected from the earth's surface during angle measurements in the vertical plane by means of metallic shields attached to the underside of the radar antenna or by earth embankments. These measures are not useful if the solid angle between direct radiation D and indirect radiation I becomes very small, which is the case if direction-finder and target have a very small height above ground. This is inherently the case when a shipboard radar for the detection of missiles flying barely above the water-surface, and also in connection with fire-control radar for antitank defense.
In addition, the prior art techniques are unsuitable for mobile applications.
The manner in which the present invention affords greatly improved target angle determination will be understood as this description proceeds.